TIME-DOMAIN APPROACH FOR THE TRANSIENT RESPONSES IN STRATIFIED VISCOELASTIC EARTH MODELS

We have developed the numerical algorithm for the computation of trans ient viscoelastic responses in the time domain for a radially stratifi ed Earth model. Stratifications in both the elastic parameters and the viscosity profile have been considered. The particular viscosity prof ile employed has a viscosity maximum with a contrast of O(10(2)) in th e mid lower mantle. The distribution of relaxation times reveals the p resence of a continuous spectrum situated between O(10(2)) and O(10(2) ) years. The principal mode is embedded within this continuous spectru m. From this initial-value approach we have found that for the low deg ree harmonics the non-modal contributions are comparable to the modal contributions. For this viscosity model the differences between the ti me-domain-and normal-mode results are found to decrease strongly with increasing angular order. These calculations also show that a time-dep endent effective relaxation time can be defined, which can be bounded by the relaxation tomes of the principal modes.